Booster spring for focal-plane shutters



- July 6,1948. R. T. JACKSON 2,444,446

BOOSTER SPRING FOR FQCAL PLANE SHUTTERS Filed D90. 11, 1945 INVENTOR. 505,557 I mos 50v ATTORNEYS.

Patented July 6, 1948 UNITED STATES PATENT OFFICE BOOSTER SPRING FOR FOCAL-PLANE SHUTTERS (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) 2 Claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes .without payment to me of any royalty thereon.

This invention relates to focal plane shutters and more particularly to a device for overcoming the initial inertia of such shutters at the time the curtain is released for making an exposure.

It is well known that the inertia of the mechanism of focal plane shutters, comprising a curtain, rollers, and other moving parts, is subject to a certain amount of inertia when the shutter is tripped due to the masses of the several elements which must be placed. in motion in an exeeedingly short period of time. Such inertia which is particularly troublesome at fast curtain speeds, in conventional shutters, causes a measurable delay in initial travel of the curtain, which results in non-uniform exposure of the film.

Accordingly, it is an object of my invention to provide means whereby curtain travel may be made substantially uniform in rate from the beginning to the end of the exposure time. It is another object of my invention to obtain this uniformity by providing a simple means readily adaptable to a conventional focal plane shutter mechanism with a minimum of alteration of the existing structure.

I accomplish the above objects by providing an auxiliary or booster spring for use in conjunction with a conventional shutter mechanism in such a manner that winding up of the mechanism places tension on the booster spring, such tension being converted into a supplemental curtain pulling force at the time the shutter is tripped.

The details of my invention are disclosed in the single figure of the drawing which shows, kinematically, pertinent parts of a focal plane shutter comprising a curtain 3, a wind-up roller 5 and a take-up roller 1. Other elements of the shutter mechanism which do not directly pertain to the operation of my device are omitted.

My invention consists in providing a pinion Ill pinned to the take-up roller 1 and rotative therewith. Pinion IO engages a gear 13 supported in any suitable manner on a fixed shaft 15. Pinion Hi and gear l3 constitute a reduction gear arrangement, and a stud I1 is secured to the face of the gear l3 and is so arranged that rotation of gear l'3 due to torque of pinion In in the direction indicated by the arrows at the time of winding up of the shutter spring causes engagement of the stud after a predetermined angular displacement with an arcuately faced lever 20 rotatively pivoted on the shaft IS. A booster spring 22 is secured between a fixed point and an end of lever 20 and is normally aligned with the lever, but is stretched by angular displacement thereof. Spring 22 is held in tension as long as roller 5 is locked at the end of the wind-up operation. However, at the time of tripping, tension in spring 22 is released, effecting a counter-rotation of lever 20 which drives stud i1 and gear [3 so as to impose a torque on pinion H! in the direction of take-up on roller 1. Thus, the tension in spring 22 provides an auxiliary driving force to roller 7 during the initial motion of curtain 3 to overcome the inertia of the several moving parts of the shutter mechanism.

I have found in at least one installation that satisfactory results are obtained by so arranging stud l1 and lever 20 that the stud engages the lever after approximately of a complete revolution of gear l3 during Wind-up operation of the shutter. Such an arrangement imparts an initial auxiliary force to the curtain for about A; travel and by the choice of a booster spring of strength suitable to bring the curtain up to proper speed just prior to commencement of exposure, a substantially uniform curtain speed even at high rate is obtainable.

It will be recognized that tension in the auxiliary spring depends on the degree of winding up of the shutter so that for various selected exposure times, as determined by the extent of winding up of the shutter, the auxiliary force imparted by the booster spring will be varied accordingly.

To suit conditions of any given focal plane mechanism the elements of my invention may be varied considerably, for example, the strength of the spring, the gear reduction ratio, the initial angular spacing of the stud from the lever, etc., are all subject to suitable design and choice for adaptation to specific cameras. Such adaptation is purely a matter of mechanical skill and would not depart from the spirit of my invention as defined in the following claims,

I claim:

1. The combination with a focal plane shutter having a take-up roller, a wind-up roller, and a curtain stretched therebetween, of booster means including a spring adapted to be stressed after a predetermined degree of rotation of said take-up roller during wind-up travel of said curtain, a pinion adapted to rotate with said take-up roller during wind-up operation of said curtain, a gear engaging said pinion and adapted to rotate about a relatively fixed axis, a lever rotatively mounted about the same axis, a pin secured to said gear and adapted to engage said lever upon a predetemined degree of rotation of said gear, said spring having a portion secured to a portion of said lever, whereby engagement of said pin with said lever stresses said spring for the purpose of providing by reaction thereof a counter-rotational force to said stud to supplement the normal force of said take-up roller upon tripping of said shutter.

2. In a focal plane shutter of the kind having a wind-up roller, a take-up roller, a curtain stretched from one of said rollers to the other, the take-up roller being windable by rolling said curtain off said take-up roller and onto said wind-up roller, the improvement which comprises means for augmenting the unwinding force of said take up roller through the first portion only of its unwinding cycle, said augmenting means consisting of a member rotatable about an axis, a booster spring having one end fixed and the other end connected to a point near the periphery of said rotatable member whereby rotation of said REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,075,751 Barberie Oct. 14, 1913 2,198,729 Junghans July 30, 1940 2,424,439 Doyle et a1 July 22, 1947 

